The present invention relates to apparatuses for controlling negative pressure in internal combustion engines. More particularly, the present invention pertains to apparatuses for controlling vacuum in internal combustion engines having brake boosters, which use vacuum to improve braking force.
In a typical engine, fuel is injected into an intake passage from a fuel injection valve to charge a homogeneous mixture of fuel and air to the associated combustion chamber. The homogeneous air-fuel mixture is ignited by an ignition plug to produce torque. This type of combustion, in which air-fuel mixture is ignited in a combustion chamber, is generally called homogeneous charge combustion. In an engine performing homogeneous charge combustion, a throttle valve located in an intake passage controls the cross-sectional area of the intake passage thereby controlling the amount of air-fuel mixture supplied to a combustion chamber. The power of the engine is controlled, accordingly. However, a high level of vacuum (a low absolute pressure) is produced by the throttling action of the throttle valve. This increases pumping losses and thus reduces the engine efficiency.
To attempt to solve this problem, stratified charge combustion has been proposed. In stratified charge combustion, a relatively rich air-fuel mixture is delivered to the vicinity of an ignition plug for ensuring the ignition of the mixture, and the generated flame burns the surrounding leaner mixture. The power of the engine is basically controlled by changing the amount of fuel injected to the vicinity of the ignition plug. This eliminates the necessity for changing the cross-sectional area of the intake passage to control the engine power, which decreases pumping loss and improves the engine efficiency. Further, stratified charge combustion allows an engine to be operated with a relatively high air-fuel ratio and thus improves the fuel economy of the engine.
Engines that switch between stratified charge combustion and homogeneous charge combustion in accordance with their running state have been proposed. One such engine has a homogeneous charge fuel injection valve and a stratified charge fuel injection valve. The homeogeneous charge injection valve uniformly disperses fuel into the combustion chamber and the stratified charge injection valve injects fuel toward the vicinity of the ignition plug. Switching to stratified charge combustion from homogeneous charge combustion when the engine load is small improves the engine efficiency and the fuel economy.
Some engines are provided with a brake booster that increases the braking force thereby decreasing the force required to depress the brake pedal. The brake booster uses vacuum, which is produced in the intake passage downstream of the throttle valve, as a drive source. That is, vacuum is communicated with the brake booster through a communicating pipe connected to the downstream side of the throttle valve. Vacuum, which corresponds to the degree of depression of the brake pedal, acts on a diaphragm incorporated in the brake booster and increases the force actuating the brake.
In such an engine, pressure in the intake passage is decreased during homogeneous charge combustion. Therefore, the vacuum available for actuating the brake booster is sufficient. However, the pressure in the intake passage is increased during stratified charge combustion. Therefore, there is less vacuum available for braking.
Japanese Unexamined Patent Publication No. 8-164840 discloses an apparatus for controlling vacuum pressure in internal combustion engines for solving this problem. In this apparatus, the throttle valve is closed when the pressure in the brake booster is greater than a predetermined reference level for temporarily decreasing the intake passage pressure, or for temporarily increasing the intake vacuum. Accordingly, sufficient vacuum for actuating the brake booster is communicated with the booster.
In the apparatus, the predetermined reference pressure level is set extremely low for guaranteeing sufficient braking force at any running state of the vehicle, that is, for generating the maximum braking force. However, the maximum braking force is only needed, for example, when a vehicle is moving at a high speed and needs to be rapidly stopped. Therefore, in the apparatus of the above publication, the throttle valve is frequently closed even if there is no need to increase the vacuum in the brake booster. The unnecessary closing action of the throttle valve temporarily decreases the amount of intake air. This reduces the fuel economy and fluctuates the engine torque thus deteriorating the engine performance.